To avoid damage to the production stratum, clear water fluids as distinguished from drilling muds are used in workover and completion operations as a standard oil well drilling practice. The clear water fluids are concentrated water solutions of common metal salts, such as NaCl, CaCl.sub.2, CaBr.sub.2, and the like. The salt selected and the concentration employed are determined by the desired density of the fluid. To obtain very high densities, zinc salts are used, such as ZnBr.sub.2 and mixtures thereof with CaBr.sub.2.
Additives are used with clear water fluids to control and increase their viscosity. Increased viscosity enhances the cuttings carrying capacity of the fluid, reduces fluid loss, and tends to promote conditioning of the formation to minimize caving and water damage to water-sensitive formation. Polysaccharide gums, such as guar gum, locust bean gum, and hydroxyethyl cellulose have been used or proposed for use in clear water workover and completion fluids. In particular, hydroxyethyl cellulose (HEC) and guar gum have been employed, but HEC is the only polysaccharide gum which has been used with some success in high density zinc salt brines.
Hydroxyethyl cellulose (HEC) and other polysaccharide gums are supplied in dry powder form. Their addition to workover and completion brines requires special mixing and dispersing equipment. HEC, guar gum, and similar additives tend to hydrate too rapidly or non-uniformly, and form clumps referred to as "fish eyes". Such fish eyes can reduce the effectiveness of the viscosifier, and, in extreme cases, cause formation damage. It has been found particularly difficult to add a viscosifier to a high density zinc brine. One procedure now used in oil field practice is to prehydrate the HEC in an alcohol-water slurry, and then slowly add the slurry to the zinc brine accompanied by turbulent mixing. This practice requires the use of a separate tank for the prehydration, and involves additional time and expense.
It is desired to develop viscosifiers which give a greater viscosity increase for a given amount of the additive, especially with reference to zinc brines. Although HEC is used with zinc brines, the viscosity effect is less than desired, and as the density of the brine increases (with increasing zinc content), the viscosity effect of the HEC additive decreases.